Method for operating a position measuring device

ABSTRACT

A system for determining a signal running time between a position measuring system and an evaluation unit. The system includes a position measuring system that has a graduation connected to a moving element, a scanning unit that scans the graduation, wherein scanning of the graduation generates signals representative of a position of the moving element and a transceiver means for generating serial data that is transmitted along a data line connected to the transceiver. The system further includes an evaluation unit connected to the data line, the evaluation unit having means for determining a signal running time between the position measuring system and the evaluation unit.

[0001] A method for the serial transmission of position data from a position measuring system to a downstream-connected evaluation unit, or electronic follow-up device, is known from EP 660 209 B1 of Applicant. In this case the bidirectional data transmission on a data line is synchronized with the aid of a phase signal of a defined transmission phase frequency on a parallel phase line. Along with an increase in the transmission of position data and possibly other data in this way, the demands regarding the transmission speed, or the amount of data to be transmitted, also increase. If now, besides the transmission speed, in addition the transmission distance between the position measuring system and the evaluation unit also increases, the result in the end are significant signal running times of the data transmitted on the data line. It is necessary to take into account the resulting delay times for the correct further processing of the various data. The reason for this is that on the part of the evaluation unit it is always necessary to provide a definite assignment of the data bits transmitted on the data line to the phase edges of the phase signal in order to assure the correct further processing of the various data. Thus, without further steps, clear limitations result with respect to maximally permissible transmission paths, or cable lengths, and the maximally permissible transmission speed, or transmission phase frequency. The correlation of the maximally permissible transmission phase frequency f_(C) and the cable length L_(K) is illustrated in FIG. 3. The graphic representation approximately indicates that with cable lengths of 100 m only a maximally permissible transmission phase frequency f_(C)=500 kHz results.

[0002] It is therefore the object of the present invention to disclose a method for operating a position measuring system which makes it possible to transmit data in serial form between the position measuring system and a downstream-located evaluation unit at high transmission speeds even over large transmission distances.

[0003] This object is attained by means of a method with the characteristics of claim 1.

[0004] Advantageous embodiments of the method in accordance with the invention ensue from the steps recited in the claims depending from claim 1.

[0005] A position measuring system suited for executing the method in accordance with the invention is the subject of claim 17.

[0006] In accordance with the invention, the specific signal running time for the specific configuration consisting of the position measuring system, transmission path and evaluation unit is now determined prior to the actual measuring operation. The signal running time determined in this way is subsequently suitably employed in the further processing of the position data and possible further data. In this way it is assured that the correct synchronization between the binary data words transmitted on the data line and the phase edges of the phase signal on the phase line can now take place with the aid of the signal running time determined in accordance with the invention. In the end it is possible in this way to assure that data can be dependably transmitted at high transmission speeds between the position measuring system and the evaluation unit even over extended transmission tracks. For example, it is now possible to transmit data at transmission phase frequencies of 4 MHz and more over cable lengths of up to 100 m which, in comparison with the prior art, represents an increase of the transmission phase frequency by a factor of 8.

[0007] The signal running time is preferably determined several times, so that it is assured that the signal running time is correctly determined, and that no erroneous value for this is determined because of possible temporary interferences on the data transmission line.

[0008] It has been shown to be particularly advantageous if the method of the invention is basically executed following each change in the configuration of the position measuring system, data transmission path and evaluation unit, and/or following every interruption of the electric power supply to the position measuring system and/or the evaluation unit. In this case the determination of the signal running time takes place automatically.

[0009] Further advantages, as well as details of the present invention, ensue from the subsequent description of an exemplary embodiment by means of the attached drawings.

[0010] Shown are in:

[0011]FIG. 1, a schematic block circuit diagram of a system consisting of the position measuring system, transmission path and evaluation unit,

[0012]FIGS. 2a to 2 e, respective signal diagrams for explaining the method in accordance with the invention,

[0013]FIG. 3, the correlation between the maximally permissible transmission phase frequency f_(C) and the cable length L_(K).

[0014] A schematic block circuit diagram is represented in FIG. 1, which illustrates the basic configuration of the system consisting of the position measuring system 10 and the downstream-connected evaluation unit 20, which are connected with each other by means of different signal lines 30.1, 30.2, 31.1, 31.2 along the transmission path. In actual use, the position measuring system 10 is arranged, for example, in a motor housing 40 and at the motor it detects position data regarding the relative and/or absolute positions of the motor rotor. The photo-electrical scanning of a measuring graduation arranged on a rotating graduated disk, for example, can be used for this. Phase-shifted scanning signals in the form of incremental signals are generated in the course of this, which are further processed by the position measuring system 10 into absolute position data and, suitably processed, are transmitted in serial form as binary data words to the evaluation unit 20 via the lines 31.1, 31.2. As for the evaluation unit 20, or electronic follow-up device, this can be a customary motor control or motor regulation, which takes over the regulation of the motor on the basis of the transmitted data.

[0015] Alternatively to such an application, the position measuring system can be employed in a machine tool, for example for determining the exact position of a tool relative to a workpiece to be processed, and for transmitting the respective position data for further processing to an evaluation unit embodied as a machine tool control. Linear measuring devices, angle encoders or angle measuring devices, for example, can be employed as position measuring systems.

[0016] Moreover, the present invention is of course not limited to the above mentioned type of the physical generation of the position data. Instead, it is also possible to operate position measuring systems based on different scanning principles.

[0017] In general, the manner of data transmission between the position measuring system 10 and the downstream-located evaluation unit corresponds to the known bidirectional, synchronously-serial data transmission in EP 660 209 B1, which is furthermore also known in the trade as the so-called EnDat® interface.

[0018] The data lines 31.1, 31.2, as well as the phase lines 30.1, 30.2, together with the transceiver modules required for data transmission, each of which is provided with the identification RS 485, are used for the actual data transmission. In the example represented, two data and phase lines 31.1, 31.2, 30.1, 30.2 each are provided, over which the phase signal and the data are transmitted in inverted form for security reasons. However, basically only one data line and one phase line would be required, therefore in what follows only a phase line and a data line are mentioned.

[0019] While the bidirectional serial transmission of the binary data words between the position measuring system and the evaluation unit takes place via the data line 31.1, 31.2, a high-frequency phase signal is transmitted on the phase line 30.1, 30.2 from the evaluation unit 20 in the direction toward the position measuring system 10 at a defined transmission phase frequency f_(C), via which in the end the entire data transmission is synchronized in a known manner.

[0020] Since, as already explained at the outset, significant data running times for the data exchanged on the data line 31.1, 31.2 result with long transmission distances, or cable lengths, and the simultaneously demanded high data transmission rates, in accordance with the invention a defined signal running time tD of the data transmitted over the phase and data lines 30.1, 30.2, 31.1, 31.2 is now determined prior to the actual measuring operation. In the example explained in what follows, the signal running time tD corresponds to the time needed by the signals to be transmitted from the evaluation unit 20 to the position measuring system 10 and back again. This signal running time t_(D) is specific for each defined configuration consisting of the position measuring system 10, transmission path and evaluation unit 20. Once the value of t_(D) is dependably known for a defined configuration, it can be taken into consideration by the evaluation unit 20 in the further processing of the data received from the position measuring system 10.

[0021] The method for determining the signal running time t_(D) in accordance with the invention will be explained in what follows by means of an example in FIGS. 2a to 2 c. The period of time prior to the actual measuring operation represented in the drawings here shows only a portion of the method of the invention.

[0022] The progression over time of the phase signal on the phase line 30. 1, 30.2 is represented in the two FIGS. 2a and 2 b. FIG. 2a shows the progression over time of the phase signal transmitted from the evaluation unit to the position measuring system, or the measuring device, starting at the time t₀, while FIG. 2b shows the progression over time of the phase signal starting at the time t₁₀ at the position measuring system. As can be seen from FIGS. 2a and 2 b, a defined chronological offset occurs between the two times t₀ and t₁₀, caused by the finite running time of the phase signal on the transmission path between the position measuring system and the evaluation unit.

[0023] It should further be noted regarding the phase signal by means of which the data transmission between the position measuring system and the evaluation unit is synchronized, that for executing the method in accordance with the invention the respective transmission phase frequency f_(C) is preferably selected on an order of magnitude f_(C)˜100 to 200 kHz. Thus, the required transmission phase frequency f_(C) for executing the method in accordance with the invention is selected to be clearly lower than the transmission phase frequency f_(C) during the actual measuring operation, which can be approximately f_(C)=4 MHz.

[0024] In principle it has been shown to be advantageous if the transmission phase frequency f_(C) for executing the method in accordance with the invention is selected to be such that the signal running time t_(D) to be expected is less than a phase period t of a phase signal with the transmission phase frequency f_(C) transmitted on the phase line:

t _(D)<1/f _(c) =t  (Equ. 1)

[0025] In the following example it is assured that, while maintaining this requirement, the detection of the signal running time t_(D) does actually take place correctly.

[0026] The signal progression over time on the phase line is represented in the two FIGS. 2c and 2 d; here, FIG. 2c shows the signal progression at the position measuring system, while the signal progression at the evaluation unit, or electronic follow-up device, is shown in FIG. 2d. In this case, too, there is again a chronological offset between the data signals at the position measuring system and at the evaluation unit, again caused by the signal running times on the transmission path. For example, at the time t₂₀ a binary data word in the form of a 6 bit mode command is sent from the evaluation unit to the position measuring system which, however, only arrives there at the time t₃₀ because of the delay.

[0027] The transmission of data, for example position data, in the form of an appropriate binary data word is now requested by the evaluation unit by via the transmitted mode command. After the appropriate data have been processed for serial transmission, the position measuring system starts with the transmission of the data in the form of a binary data word at the time t₄₀. Because of the resulting signal running time over the transmission path, the respective data word arrives at the evaluation unit only at the time t₅₀.

[0028] Now the time between the time t₅₀ and the last rising phase edge of the phase signal on the phase line is defined as the signal running time t_(D,i) caused by the transmission path. In the present example, a corresponding rising phase edge existed at the evaluation unit for the last time prior to detecting the start bit at the time t₃₅.

[0029] For determining the running time t_(D,i), a counter is started for the first time at the evaluation unit, such as is illustrated in FIG. 2e; in the block circuit diagram in FIG. 1, the counter is identified schematically by the reference number 21. The counter counts upward at a preset counting frequency f_(Z), wherein the selected counting frequency f_(Z) has been selected to be clearly greater than the transmission phase frequency f_(C) of the phase signal on the phase line used during the measuring operation. In this case the counting frequency f_(Z) is preferably selected to be at least eight times as great as the transmission phase frequency f_(C) used during measuring operations, i.e.

f _(Z)>8*f _(C)  (Equ. 2)

[0030] If, for example, a transmission phase frequency f_(C)=4 MHz is intended for the measuring operation, the counting frequency f_(Z)=32 MHz is selected.

[0031] To assure a sufficiently exact and interference-proof determination of the signal running time t_(D), the selected counting frequency f_(Z) may maximally vary by +/−10%. Accordingly, in the above example with f_(Z)=32 MHz, a maximum frequency variation of +/−3.2 MHz could be tolerated.

[0032] Now the counter, which was first started at the time t₃₁ with a rising switching edge of the phase signal on the phase line, counts up until it is either started again by a new rising switching edge of the phase signal, or the actual count Z of the counter is detected, or stored, by the rising phase edge of the arriving start bit of the transmitted data word. Accordingly, in the example represented, the counter in FIG. 2e is started again at the times t₃₁ to t₃₅ by the rising switching edge of the phase signal; at the time t₅₀, the actual count Z of the counter is detected and stored because of the rising phase edge of the arriving start bit of the data word transmitted by the position measuring system.

[0033] Thereafter, the signal running time t_(D,i) can be determined by the evaluation unit from the known counting frequency f_(Z) and the detected count of the counter, i.e. t_(D,i)=Z*1/f_(Z).

[0034] The signal running time t_(D,i)(i=2) is preferably determined at least one more time in this way, and an average signal running time t_(D) is determined from the individual signal running times t_(D,1), t_(D,2). In the present example of the method of the invention, the signal running time t_(D,i)(i=1, 2, 3) is determined a total of three times in the explained way, and subsequently the average signal running time tD is determined as the arithmetic mean of the individual signal running times t_(D,i), i.e.

t _(D)=(t _(D,1) , t _(D,2) , t _(D,3))/3   Equ. (3)

[0035] The repeated determination of the signal running times tD,i assures that possible errors in their determination and a consistency check of the detected individual values tD,i can be removed. Within the scope of the invention it is of course also possible to determine more than three individual values tD,i and to derive therefrom an average signal running time t_(D) by forming the arithmetic mean.

[0036] It has been proven to be advantageous if, following the determination of the mean from the individual signal running times t_(D,i), a further check is made by what amount Δt_(D,i) the individual signal running times t_(D,i) deviate from the average signal running time t_(D). For example, it is possible to predetermine that the maximally permissible deviation Δt_(D,i) of a single determined signal running time t_(D,i) from the mean t_(D) is allowed to be maximally ⅛ of the phase length T of the transmission phase frequency f_(C) during measuring operations, i.e.

Δt _(D,i)<(1/8)*(1/f _(C))  Equ. (4)

[0037] In case of an impermissible greater deviation, an error message would be issued by the evaluation unit, and the determination of the signal running times t_(D,i) would be repeated, etc.

[0038] After the signal running time tD has been determined in this way, this value can be taken into consideration during the following processing by the evaluation unit of the data received from the position measuring system.

[0039] Finally, a further embodiment of the serial data transmission between the position measuring system and the downstream-located evaluation unit will be explained. This process was shown to be advantageous in particular with long transmission paths and the signal running times resulting therefrom, if the signal running time t_(D) had previously been determined in a manner in accordance with the invention.

[0040] Thus, during measuring operations it is provided here in the course of each request for data from the position measuring system to initially see to it that the position measuring system no longer transmits data in the direction toward the evaluation unit. It is intended in this way to prevent an otherwise possible collision on the data line. To this end, in case of a data request first a delay signal at the logical LOW level, which has a defined preset length of time t_(ST) is transmitted on the data line from the evaluation unit to the position measuring system. The actual phase signal of the respective transmission phase frequency f_(C) is transmitted and the data transmission is performed in the usual manner only after this length of time t_(ST). Accordingly, the first period of the transmitted phase signal is composed of the transmission of the delay signal with the logical LOW level and the length t_(ST) and a subsequent logical HIGH level of the transmission phase length t/2. The length t_(ST) of the delay signal is selected to be, for example, t_(ST)=1.5 μs.

[0041] Further embodiment variations of the method in accordance with the invention of course exist besides the explained example. 

1-17 (canceled)
 18. A method for operating a position measuring system, downstream of which an evaluation unit for further processing position data is arranged, the method comprising: requesting serial transmission of data by said evaluation unit from said position measuring system on a data line, which connects said evaluation unit and said position measuring system, wherein said data is transmitted in a direction toward said evaluation unit; starting a counter at said evaluation unit at a definite starting time, which counts up at a predetermined counting frequency; determining an actual count of said counter at a certain time as soon as said data are detected by said evaluation unit; and determining a signal running time between said position measuring system and said evaluation unit from said determined actual count of said counter.
 19. The method in accordance with claim 18, further comprising: determining at least one additional signal running time; and determining an average signal running time from said signal running time and said at least one additional signal running time.
 20. The method in accordance with claim 18, wherein said determining said signal running time takes place prior to a measuring operation performed by said position measuring system.
 21. The method in accordance with claim 20, wherein said predetermined counting frequency is selected to be clearly higher than a transmission clock frequency, used in said measuring operation, of a clock signal on a clock line connected to said evaluation unit and said position measuring system, by which said serial transmission of said data on said data line is clocked.
 22. The method in accordance with claim 21, wherein said predetermined counting frequency is at least eight times greater than said transmission clock frequency used in said measuring operation.
 23. The method in accordance with claim 21, wherein said predetermined counting frequency has a maximal frequency variation of +/−10%.
 24. The method in accordance with claim 18, wherein said counter is restarted by a switching edge of a clock signal on a clock line, which is connected to said evaluation unit and said position measuring system, over which said serial transmission of said data on said data line is clocked.
 25. The method in accordance with claim 24, wherein said switching edge is a rising switching edge.
 26. The method in accordance with claim 19, wherein said at least one additional signal running time are two additional signal running times, and thereafter said average signal running time is determined as an arithmetic mean from said signal running time and said two additional signal running times.
 27. The method in accordance with claim 19, further comprising: subsequent to said determining said average signal running time, checking deviations of said signal running time and said at least one additional signal running time from said average signal running; and issuing an error message in case of an impermissibly large deviation.
 28. The method in accordance with claim 18, wherein position data in the form of binary data words are transmitted as said data for determining said signal running time, and said actual count of said counter is detected as soon as a rising or falling edge of a start bit of one of said binary data words is detected by said evaluation unit.
 29. The method in accordance with claim 21, wherein in the course of said determining said signal running time said transmission clock frequency on said clock line is selected to be clearly lower than said transmission clock frequency on said clock line during said measuring operation of said position measuring system.
 30. The method in accordance with claim 29, wherein said transmission clock frequency on said clock line during said determining said signal running time is selected to lie between 100 and 200 kHz.
 31. The method in accordance with claim 29, wherein said transmission clock frequency on said clock line during said determining said signal running time is selected in accordance with the following condition: t_(D)<1/f_(C), wherein t_(D)=signal running time and f_(C)=transmission clock frequency.
 32. The method in accordance with claim 18, wherein said determining said signal running time is performed following every change of the configuration of a system defined by said position measuring system, a path of transmission of said data and said evaluation unit.
 33. The method in accordance with claim 32, wherein said determining said signal running time following every change of configuration takes place automatically.
 34. The method in accordance with claim 18, wherein said determining of said signal running time is performed following any interruption of a supply of electric power to said position measuring system and/or said evaluation unit.
 35. The method in accordance with claim 34, wherein said determining said signal running time following any interruption of a supply of electric power takes place automatically.
 36. The method in accordance with claim 20, further comprising transmitting, during said measuring operation and in case of a data request, a delay signal of a logical LOW level of a defined length of time from said evaluation unit to said position measuring system.
 37. The method in accordance with claim 18, wherein said determined value of said signal running time is taken into consideration during processing of said data received by said evaluation unit from said position measuring system.
 38. A system for determining a signal running time between a position measuring system and an evaluation unit comprising: a position measuring system comprising: a graduation connected to a moving element; a scanning unit that scans said graduated disk, wherein scanning of said graduation generates signals representative of a position of said moving element; and a transceiver means for generating serial data that is transmitted along a data line connected to said transceiver; and an evaluation unit connected to said data line, said evaluation unit comprising means for determining a signal running time between said position measuring system and said evaluation unit.
 39. The system for determining a signal running time in accordance with claim 38, wherein said means for determining a signal running time comprises a counter which counts up at a predetermined counting frequency.
 40. The system for determining a signal running time in accordance with claim 38, further comprising means for determining an average signal running time from said signal running time and at least one additional signal running time.
 41. The system for determining a signal running time in accordance with claim 38, further comprising a clock line connected to said position measuring system and said evaluation unit.
 42. The system for determining a signal running time in accordance with claim 39, further comprising a clock line connected to said position measuring system and said evaluation unit.
 43. The system for determining a signal running time in accordance with claim 41, wherein said counter is restarted by a switching edge of a clock signal on said clock line.
 44. The system for determining a signal running time in accordance with claim 43, wherein said switching edge is a rising switching edge.
 45. The system for determining a signal running time in accordance with claim 40, further comprising: means for determining deviation of a signal running from said average signal running; and means for issuing an error message in case of an impermissibly large deviation.
 46. The system for determining a signal running time in accordance with claim 38, wherein said moving element is a rotor of a motor and said graduation comprises a rotating graduation disk that is coupled to said rotor.
 47. A method for determining a signal running time between a position measuring system and an evaluation unit, said position measuring system and said evaluation unit being connected by a data line and a clock line, the method comprising: requesting serial transmission of data from said position measuring system by said evaluation unit, wherein said requesting is associated with a defined edge of a clock signal transmitted on said clock line from said evaluation unit to said position measuring system; transmitting said requested data in the form of at least one binary data word from said position measuring system to said evaluation unit, wherein said at least one binary data word is transmitted on said data line; and determining a signal running time from a time difference between said defined edge of said transmitted clock signal on said clock line and an edge of a signal on said data line indicating arrival of said at least one binary data word transmitted to said evaluation unit.
 48. The method in accordance with claim 47, wherein said time difference is determined with a counter which counts up at a predetermined counting frequency.
 49. The method in accordance with claim 48, further comprising: starting said counter with a defined edge of said clock signal on said clock line; and stopping said counter with a defined edge of a signal on said data line.
 50. The method in accordance with claim 47, wherein said determining said signal running time takes place prior to a measuring operation performed by said position measuring system.
 51. A system for determining a signal running time between a position measuring system and an evaluation unit comprising: a position measuring system comprising: a graduation connected to a moving element; a scanning unit that scans said graduated disk, wherein scanning of said graduation generates signals representative of a position of said moving element; and a first transceiver means for generating serial data that is transmitted along a data line connected to said first transceiver; a second transceiver means for generating a clock signal that is transmitted along a clock line connected to said second transceiver and an evaluation unit connected to said data line and said clock line, said evaluation unit comprising means for determining a signal running time between said position measuring system and said evaluation unit based on a time difference between a defined edge of a clock signal transmitted on said clock line and an edge of a signal on said data line indicating arrival of at least one binary data word transmitted to said evaluation unit.
 52. The system for determining a signal running time in accordance with claim 51, wherein said means for determining a signal running time comprises a counter which counts up at a predetermined counting frequency.
 53. The system for determining a signal running time in accordance with claim 52, wherein said counter is started with said defined edge of said clock signal on said clock line and is stopped with a defined edge of said signal on said data line.
 54. The system for determining a signal running time in accordance with claim 51, wherein said determining said signal running time takes place prior to a measuring operation performed by said position measuring system.
 55. The system for determining a signal running time in accordance with claim 51, wherein said moving element is a rotor of a motor and said graduation comprises a rotating graduation disk that is coupled to said rotor. 